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bromide
Microporous and Mesoporous Materials 79 (2005) 205–214 www.elsevier.com/locate/micromeso
Bromine mediated partial oxidation of ethane over nanostructured zirconia supported metal oxide/bromide Aysen Yilmaz a, Xiao-Ping Zhou a, Ivan M. Lorkovic a,*, Gurkan A. Yilmaz a, Leroy Laverman a, Mike Weiss b, Shouli Sun b, Dieter Schaefer c, Jeffrey H. Sherman b, Eric W. McFarland c, Peter C. Ford a,*, Galen D. Stucky a,* a
Department of Chemistry and Biochemistry, University of California, Santa Barbara, CA 93106, USA b GRT Inc., 861 Ward Dr., Santa Barbara, CA 93111, USA c Department of Chemical Engineering, University of California, Santa Barbara, CA 93106, USA Received 8 September 2004; received in revised form 3 November 2004; accepted 3 November 2004 Available online 6 January 2005
Abstract Partial oxidation of ethane to an array of products was achieved through use of bromine/supported metal bromide as redox mediator for oxygen activation. In a two stage flowing fixed bed reactor, ethane bromination (350–450 C) to EtBr and HBr was followed by metathesis of these intermediates with supported cobalt or copper oxides (200–250 C) to produce ethanol (up to 64% selectivity at 45% EtBr conversion), diethyl ether (64% selectivity at 44% conversion), and ethylene (26% selectivity at 68% conversion at 200 C, but conversions and selectivities approaching 100% at temperatures >300 C) as major products, and butadiene, vinyl bromide, acetaldehyde, ethyl acetate, and acetone as minor products. The supported cobalt and copper bromides thus formed were reoxidized with flowing O2 to liberate bromine and this cycle was repeated at least three (and as many as fifty) times to establish the reproducibility of both the metathesis product distribution and of the oxide delivery and bromide sequestration capacity of the metal oxide/metal bromide mediators. Different formulations of the supported and unsupported metal oxide/metal bromide were investigated; zirconia or titania-supported Cu and Co oxides prepared from oxalic acid gels showed higher and more reproducible oxide/bromide exchange activity and capacity. These materials performance is discussed in terms of their morphology, crystallinity, and porosity at different states of bromination in the partial oxidation cycle. 2004 Elsevier Inc. All rights reserved. Keywords: Ethane; Partial oxidation; Bromine mediated; Supported metal oxide; Solid reactant; Ethanol derivatives
1. Introduction Direct conversion of light alkanes in natural gas feedstock to liquid products is an attractive research area in the context of more economical feedstock transportation, development of more energy efficient methods for *
Corresponding authors. Tel.: +1 805 893 5239; fax: +1 805 893 4120. E-mail addresses: [email protected], [email protected] (I.M. Lorkovic), [email protected] (P.C. Ford), stucky@chem. ucsb.edu (G.D. Stucky). 1387-1811/$ - see front matter 2004 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2004.11.003
the conversion of light alkanes to commodity chemicals, as well as towards minimization of greenhouse gas emission [1]. We have previously reported a novel process for functionalization of alkanes by bromine mediated partial oxidation [2,3] and have also shown how this technique could be used to produce higher hydrocarbons from methane [4], and olefin oxides from olefins [5]. Ethane is the second most abundant hydrocarbon component of natural gas and is itself a valuable feedstock for ethylene production through steam cracking [6]. Derivatives of ethylene include ethanol, from solid acid catalyzed hydration, acetaldehyde and its
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condensation products, from Wacker chemistry, ethylene oxide, and acetic acid. Ethanol production from biomass fermentation provides a large source of ethanol as well [7]. Work directed towards ethanol production from direct ethane oxidation has shown promise but thus far only low conversions and selectivities have been demonstrated [8]. We describe herein the application of our bromine/metal oxide mediated technique for partial alkane oxidation to the challenge of direct ethane conversion to ethanol and other derivatives of ethane (Eqs. (1)–(4)), Scheme 1. MBr2 þ 1=2 O2 ! MO þ Br2
ð1Þ
C2 H6 þ Br2 ! C2 H5 Br þ HBr
ð2Þ
C2 H5 Br þ HBr þ MO ! MBr2 þ EtOH
ð3Þ
Overall : C2 H6 þ 1=2O2 ! EtOH
ð4Þ
Selectivity is a key parameter in any chemical process. For methane and ethane partial oxidation all C–H bonds are equivalent and hence the main goal is to perform only a ‘‘2 electron’’ oxidation on the substrate and to avoid overoxidation, leaving the other C–H bonds intact. In this sense, ethane bromination is a particularly effective means of selective ethane oxidation, as the rate of secondary bromination to the two isomers of C2H4Br2 is slightly slower [9] than the first bromination under mild conditions, so 90% selectivity of bromoethane product is possible at 25% conversion (Eqs. (1) and (5)). C2 H5 Br þ Br2 ! HBr þ ðxÞ1; 1 C2 H4 Br2 þ ð1 xÞ1; 2 C2 H4 Br2
ð5Þ
Further developments in catalytic electrophilic bromination are also anticipated to improve the selectivity for monobromination beyond that achievable using free radical chemistry. For example, Akhrem et al. have re-
EtH T1 Br2 EtBr + HBr M(O) T2
MBr2
T3 1/2 O2
EtOH Et2O, C2H4, etc. Scheme 1. Metal bromide/metal oxide mediated ethane partial oxidation.
ported that CBr4 Æ 2AlBr3 is a reactive electrophilic catalyst for ethane bromination under mild conditions (60 C) [10]. To complete the transformation to usable products, however, bromide equivalents in HBr and alkyl bromide must be exchanged for oxide, and bromine mediator subsequently recovered. The advantages of bromine over other halogens in this alkane partial oxidation scheme can be related to the favorable reduction potential of Br2 in relation to O2. Alkane bromination is only mildly exothermic (while iodination is ergoneutral and does not go to completion) and therefore more straightforward to perform on a large scale than alkane fluorination and chlorination. Br2 recovery by metal bromide reaction with O2 is also spontaneous for a wider array of materials formulations than corresponding chlorine recovery from chlorides because of the less positive reduction potential of Br2 (1.07 V vs NHE) in comparison to Cl2 (1.37 V) and O2 (1.23 V). In this scheme, there are two sources of undesirable deeper substrate oxidation: secondary bromination in the alkane activation step (Eq. (5)) and redox activity of the metathesis material. The first of these overoxidations gives a mixture of dibromoethane isomers that have unique properties in their metathetical reactivity with metal oxides. 1,1-dibromoethane leads almost invariably to the HBr elimination product vinyl bromide upon reaction [11], although metathesis products such as acetaldehyde are possible at below 250 C. 1,2-dibromoethane, on the other hand, tends to undergo disproportionation to CO2 and more reduced products such as ethylene [12]. This secondary bromination product distribution may be controlled to favor 1,1- or 1,2dibromoethane (vide infra) [13]. The other source of substrate overoxidation is metal oxide redox reactivity within the redox potential range (1.2 V) experienced by the solid during cycles of metathesis and regeneration. This range is defined by exposure to alkane derivatives during metathesis (reducing atmosphere), and to O2 during regeneration (oxidizing atmosphere). If the oxidation state of the solid changes within this potential range, deeper oxidation of ethane derivatives and more oxygen consumption during regeneration is expected. Such deep oxidation does indeed occur over supported copper and cobalt oxides tested here, and in general for first row transition metal oxides, vanadium and higher. Furthermore, the more reactive partially oxidized intermediates are degraded more rapidly than ethane substrate. An example of the redox reactivity expected for many supported transition metal oxides is shown below; in this case a two electron oxidation of ethanol to acetaldehyde coincides with metathesis (Eqs. (6) and (7)). Butadiene, acetone, and ethyl acetate are also observed as side products. 3MBr2 þ 2O2 ! M3 O4 þ 3Br2
ð6Þ
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207
3C2 H5 Br þ 3HBr þ M3 O4 ! 3MBr2 þ 2EtOH þ C2 H4 O þ H2 O
ð7Þ
We characterize in this study the parameters governing the conversion and selectivity of metal bromidemediated ethane partial oxidation. First we describe thermal ethane bromination product distributions as a function of reactor temperature, space-time and packing. Subsequently we examine oxygenated product distributions obtained upon treatment of the intermediate brominated ethane stream with reactive and regenerable supported copper and cobalt metal oxides. These solids are unique in their ability to sequester and neutralize stoichiometric amounts of bromide, and quantitatively release bromine upon oxygenation at temperatures <350 C over at least 50 cycles. Finally we describe the oxide and bromide morphologies as characterized by XRD, SEM, BET, and TGA, to attempt to gain insight into the favorable reactivity of this unique set of sol–gel prepared solids. For EtBr/HBr metathesis reactivity, equivalent amounts of unsupported cobalt and copper oxides, cobalt and copper oxides wet impregnated on high surface area zirconia supports, as well as other sol–gel preparation routes gave materials with similar HBr neutralization capacity but less metathesis reactivity with EtBr.
2. Experimental 2.1. Reactor system and reaction conditions The reactor system used for ethane oxidation reactions is composed of two vertical cyclindrical reactors in series, Scheme 2 (the first either 1 or 0.3 cm diam. · 10 cm len. and the second 1 · 10 cm). The feed to the first reactor is an ethane/bromine mixture of variable ratio (between 10:1 and 3.2:1 Ethane:Br2) which reacts either in an empty heated tube or in a packed bed of 50 mesh borosilicate beads. The second reactor contains the supported metal oxide fixed bed, reactive toward oxide/bromide exchange. Samples were taken for GC analysis at any location; upstream of the first reactor (blank), between the first and second reactors (ethane bromination products), or after the second reactor (products of overall reaction). Independent reactions of monobromoethane and dibromoethanes over metal oxides were also performed in the second stage of the reactor system. All reactions were carried out at atmospheric pressure. A mass flow controller (Matheson) was used to control the flow rate of ethane. Bromine was introduced into the ethane stream by a temperature controlled (ice water for P(Br2) = 70 Torr, 20 C bath for P(Br2) = 180 Torr) bromine bubbler equipped with a bypass valve to allow controlled entrainment of bromine into the ethane gas feed. Metathesis of EtBr over
Scheme 2. Laboratory scale reactor flow diagram for bromine mediated partial oxidation of ethane (dimensions not to scale).
metal oxide formulations was studied over a temperature range of 150–350 C. at gaseous space velocities between 0.02 and 0.1 h1 [(g feed)(g solid)1 h1]. Reactive metal oxides were typically exhausted to 15–25% of their bromine carrying capacity [3] before regeneration. 2.2. Synthesis of metal oxides and characterizations The supported metal oxides described in this study, MxZryOz and KwMxZryOz (where the molar ratio of M to Zr is 1:1, and the molar ratio of K to M to Zr is 1:4:5), as well as the corresponding titanates, were prepared from aqueous solutions of metal nitrates (0.5 M) thoroughly mixed with a solution of zirconium tetrapropoxide (or titanium isopropoxide) in aqueous oxalic acid (1.0 M). After evaporation of the solvent at 110 C, the mixture was calcined at 500 C for 4 h. After cooling, the solid was ground in a mortar and stored for further use. For data reported here, the bromination reoxidation cycle was performed until the product distribution did not change from run to run (usually 3 cycles). For purposes of comparison, zirconia supported cobalt and copper oxides were prepared using the same preparation above, except using a citric acid (1.0 M as above) based zirconia gel. Also tested were cobalt oxide supported on an equal weight of preformed zirconia/silica (Grace-Davison ‘‘Davicat’’ SIZR 4700 zirconia on amorphous silica, 368 m2/g), prepared by wet impregnation/calcination (500 C). Finally, comparable beds of
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unsupported Co3O4 admixed with enough 50 mesh glass beads to achieve the same bulk bed volume as the other samples were prepared. X-ray powder diffraction patterns were obtained on a Scintag PADX diffractometer using Cu-Ka radiation at 45 kV and 35 mA. The crystalline domain sizes were determined from powder XRD patterns and scanning electron microscopy (SEM) images. Environmental scanning electron microscope images were carried out with a Philips XL-30 ESEM-FEG. The EDS system (Princeton Gamma Technology) was equipped with an intrinsic germanium detector with a resolution of 115 eV and an ultra-thin window for light element detection and used IMIX version 10.594 software. Gas adsorption data for BET surface area determinations and pore diameter measurements were obtained with a Micromeritics ASAP-2000 porosimeter. XPS measurements were carried out using a Kratos Axis Ultra system. Monochromatic Al-Ka radiation was used. The anode was operated at 8 kV and 30 mA and the pressure in the analysis chamber was about 2 · 106 Pa. The binding energies (BE) were corrected for charging by assigning the C(1s) signal to 284.6 eV. Elemental analysis was performed by Galbraith Laboratories in Knoxville, TN. 2.3. Regeneration conditions Regeneration of the supported metal oxide from the spent metal bromide was performed by passing oxygen at 320 C through the reactor which contained the supported metal bromide. Evolved bromine was trapped as sodium hypobromite in a 2 M NaOH trap and analyzed by UV (HP-8452A diode array UV–Vis Spectrophotometer, e(330 nm) = 301 M1 cm1) [14]. During regeneration, the degree of utilization of O2 was estimated to be >90% by monitoring the non-bromine gas efflux rate, which completely stopped until all the metal bromide was converted into oxide. 2.4. Analysis of products Ethane conversion was measured by GC. The output stream of the bromination reactor was passed through an aqueous alkaline trap to remove HBr, and mass flow controlled dimethyl ether internal standard and nitrogen diluent were added just upstream of the GC injection valves. Ethane bromination product selectivities were measured by 1H-NMR (Varian 200 MHz and 400 MHz, delay time between pulses >20 s), by passing the output of the first reactor through a CDCl3 trap (typically 6.0 g for trapping 2 mmol products, quantified by comparison to known CHCl3 internal standard). While significant ethane, ethylene, and HBr were lost to headspace by this technique, brominated hydrocarbons were trapped quantitatively.
For tandem bromination/metathesis experiments, gaseous products exiting the second reactor, which contained the HBr scrubber/oxygen bromine exchange medium, were diluted with a nitrogen stream and passed through a dead volume (200 ml) and analyzed by an HP-6890A type GC equipped with DB-FFAP, Porapak-Q 80/100, and HP-Plot Q columns connected to FID, TCD, and mass selective (HP-5973) detectors, respectively. Alternatively, these products, which included bromoethane, ethanol, diethyl ether, ethyl acetate, acetaldehyde, and 1,3-butadiene were collected in a cooled (dry ice/acetone) CDCl3 trap, and analyzed by 1H-NMR as described above.
3. Results 3.1. Thermal ethane bromination reaction The reaction of ethane with bromine has been studied by numerous researchers [9,15–19]. The reaction is propagated by bromine atoms reacting with ethane to give HBr and by ethyl radicals reacting with bromine to give ethyl bromide and bromine atoms. As can be seen in Table 1, the selectivity to ethyl bromide at 25% conversion in thermal ethane bromination experiments is relatively invariant (85–90%) under a variety of conditions between two minutes and one second space time and 300 C to 450 C. Secondary bromination of ethyl bromide is slightly slower than ethane bromination [9] and gives 1,1- and 1,2-dibromoethane, which have similar thermodynamic properties [20]. Because the 1-yl-bromoethyl radical is more stable than the 2-yl-bromoethyl radical by approximately 5 kcal/mol [21], however, 1,1-dibromoethane is the kinetically favored product in a gas phase free radical reaction. Our empty reactor results are consistent with this picture, and 1,1-dibromoethane is the dominant dibromoethane isomer by over a factor of ten, Table 1. With the addition of seemingly innocuous glass beads, however (400 lm, 40–60 mesh, McMaster-Carr sandblasting media), the secondary bromination product distribution shifts dramatically to favor 1,2-dibromoethane over 1,1-dibromoethane by over a factor of ten. If this isomerization reaction is catalyzed by the bead surface, this result is surprising considering the supposed thermodynamic equivalence of these species [22], and the expectation that such an isomerization should be reversible. We rule out the possibility that the glass beads are catalyzing the secondary bromination because if such bromination catalysis were competitive with the gas phase reaction, the overall selectivity to dibromoethane products would be expected to increase in the absence of any similar enhancement in the initial primary bromination step. To wit, no increase in the overall rate of bromination is observed in the
A. Yilmaz et al. / Microporous and Mesoporous Materials 79 (2005) 205–214
209
Table 1 Ethane bromination: ethane conversion and selectivity to ethyl bromide, 1,1-dibromoethane and 1,2-dibromoethane as a function of temperature, reactor space time, and bed packing Temperature (C)
450 450 400 400 375 350 350 300 300 280
Space time (s)
1 3 1 3 3 27 27 29 29 230
Packing
Conversion (%)
None None None None None None Borosilicate beads None Borosilicate beads None
Selectivity (%)
23.5 25.0 23.0 25.0 24.9 25.5 27.7 26.0 25.7 25.9
EtBr
1,1-DBE
1,2-DBE
90.4 89.3 91.2 89.4 87.5 86.5 85.2 87.3 87.8 82.1
8.8 8.9 7.7 9.2 10.0 12.8 1.2 12.0 1.7 15.6
0.8 1.8 1.1 1.4 2.5 0.7 12.2 0.7 10.3 2.3
All data represent 100% bromine conversion. EtH flow rate is typically 5 sccm and bromine is added by entrainment through a temperature controlled bromine bubbler.
Table 2 Bromine/cobalt oxide mediated ethane partial oxidation product distribution: dependence on supported cobalt oxide method of preparation EtBr conversion (%)
Et2O (%)
EtOH (%)
Other products
Bulk Co3O4 CoO wet impreg. on SiO2/ZrO2 Citrate sol/gel
<5 25
0 15
0 2
50
30
7
Oxalate sol/gel
43
27
7
2% 4% 3% 4% 3% 4% 3%
ViBr ViBr C2H4 ViBr C2H4 ViBr C2H4
For each entry, a cylindrical 8 (length) · 1 cm (diameter) plug of solid representing 25 mmol of Co was used as the oxide/bromide metathesis reagent. Ethane flow was 5 sccm and the Br2 flow was 0.5 sccm, giving a total exposure of the solid to 6.2 mmol bromide both as HBr and R–Br. The integrated product output over 5 h is summarized. All samples shown quantitatively and stoichiometrically neutralize streams containing HBr, producing either supported or unsupported CoBr2 and water. The product outputs shown represent results of the third cycle or higher of metathesis/ regeneration. Selectivity of all products is calculated in terms of converted ethane.
presence of the glass beads. We are currently pursuing the source of these effects further, and exploring the scope and limitations of this isomerization reactivity [13]. 3.2. Sequential zone flow reactor (SZFR) experiments The three step bromine and metal oxide mediated oxidation scheme was tested using ethane bromination to generate the feed that was then passed into metal oxide bed for neutralization/metathesis. To this end, ethane at 15 psia was entrained with bromine in a bubbler immersed in ice and the mixture passed through a bromination reactor as described above to generate a stream of EtBr, HBr, and trace dibromoethane in excess ethane carrier. This stream was directly passed into the metathesis reactor for oxide/bromide exchange with the solid oxide, and the output evaluated by periodic GC analysis. The effect of the support and preparation conditions on the capacity, reactivity, and regenerability of cobalt oxides as metathesis reagents was evaluated by comparing the behavior of bulk Co3O4 to that of supported (wet
impregnation) and sol–gel prepared samples, Table 2. While all samples are capable of neutralizing HBr and releasing Br2 upon treatment with O2 over several cycles, the supported cobalt oxide, prepared by both wet impregnation and sol–gel routes, are reactive in oxide bromide exchange with both EtBr and trace dibromoethane feeds. Despite the higher surface area of the commercially available silica/zirconia support, and the high reactivity of such supported materials reported by Ying and coworkers [23] the most reactive materials for EtBr metathesis are those prepared by the sol–gel route, with both citric and oxalic acid gels showing comparable reactivity. Figs. 1 and 2 show the time dependent product distribution for the two stage reaction representing both bromination (375 C) and metathesis, using a bed of sol–gel zirconia supported cobalt oxide and copper oxide, respectively. The bromine to ethane feed ratio is 1:10. The major product for this reaction for both oxides is diethyl ether (Figs. 1, 2), and there is significant breakthrough of bromoethane reactant (but not of HBr) at a point well below the calculated bromine neutralizing capacity of the solid. Other oxygenated products include
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CuZrO3 1.6 1.4
1
C2 in
mmoles/h
Et2O C2H4
0.6
EtBr ViBr
0.4
mmol/h
1.2 C2 in EtOH
0.8
1
EtOH Et2O
0.8
C2H4 AcH EtOAc
0.6
EtBr CO2
0.4
CO2
0.2
0.2
0 0.00
0 0.00
1.00
2.00
3.00
4.00
5.00
6.00
time, hours 1.00
2.00
3.00
4.00
5.00
6.00
time, hours Fig. 1. Ethane partial oxidation by sequential bromination (375 C, EtH:Br2 = 10) and oxide/bromide metathesis (200 C) over oxalic acid sol–gel derived zirconia/cobalt oxide composite (5 g). Ethane flow rate (residence time within metathesis bed) is 5.0 sccm (0.7 min).
ethanol, acetone, acetaldehyde, ethyl acetate and CO2, while elimination products include ethylene, vinyl bromide, and butadiene. The supported copper oxide is more selective to ethanol than the supported cobalt oxide (Fig. 3). Butadiene production was unique to the cobalt oxide. CuZrO3, in addition to evolving more CO2 and ethylene, also produced more ethyl acetate and acetone. The production of CO2 dropped sharply after the first hour of the reaction and stayed constant after Cu(II) was reduced to Cu(I). The selectivity of all major products is given in Fig. 3 (grey and striped gray bars). Carbon material mass balance analysis for the reaction over the metal oxides indicated that some of the reaction products (5–15% depending on the solid and degree of EtBr exposure) were adsorbed to the surface. Although diethyl ether hydrolysis to ethanol downstream from the metathesis reactor is a feasible path to
Fig. 2. Ethane partial oxidation by sequential bromination (375 C, EtH:Br2 = 10) and oxide/bromide metathesis (200 C) over oxalic acid sol–gel derived zirconia /copper oxide composite (5 g). Ethane flow rate (residence time within metathesis bed) is 5.0 sccm (0.7 min). The spikes of ethanol are due to condensed aqueous ethanol entering the evaporation dead volume, upstream from the GC, which serves to dampen and allow quantitation of resulting short intervals of high ethanol reactor efflux.
ethanol formation, ether itself is not as desirable a product. In an effort to reduce the ether output of EtBr/HBr metathesis reactor, other solids and dopants were investigated. Fig. 3 shows the metathesis product output of undoped and potassium-doped cobalt and copper zirconates. The potassium-doped solid reactivity is approximately the same as the undoped materials, but the product distribution was significantly altered to favor EtOH over other products. Fig. 4 shows the temporal product distribution of the two stage bromine mediated ethane partial oxidation using potassium doped supported cobalt oxide at two slower flow rates, run to the same oxide conversion. Significantly, for these solids the EtBr breakthrough is observed at approximately the same level of solid utilization (5%), and is independent of the feed flow rate, Fig. 4, suggesting the gas/solid equilibrium idealized in Eqs. (8) and (9).
Fig. 3. Steady state (stabilized over several cycles) product distribution observed for oxalic acid sol–gel derived zirconia supported copper and cobalt oxides, unpromoted or promoted with potassium. Conditions are identical to those used in Figs. 1 and 2.
A. Yilmaz et al. / Microporous and Mesoporous Materials 79 (2005) 205–214
2 cc/min EtH
mmol C2/h
1 C2 in
0.8
EtOH Et2O
0.6
C2H4 EtBr
0.4
ViBr CO2
211
of potassium and basicity of associated anions, these basic counterions (bromide/oxide) are responsible for deactivating electrophilic sites of the undoped metal zirconate. Ethylene production, on the other hand, is not as strongly affected by potassium addition as ether formation, suggesting that ethylene may form at either electrophilic or basic sites on the oxide. In an additional twist, the ether output of the citrate based cobalt/zirconia gels was not affected by the addition of potassium.
0.2 0 0.00
3.3. Characterization of metal oxides 2.00
4.00
6.00
8.00
10.00
12.00
time, h 1 cc/min EtH
mmol C2/h
1 C2 in EtOH Et2O C2H4
0.8 0.6
EtOAc EtBr ViBr CO2
0.4 0.2 0 0.00
5.00
10.00
15.00
20.00
time, h Fig. 4. Ethane partial oxidation by sequential bromination (375 C, EtH:Br2 = 10) and oxide/bromide metathesis (225 C) over oxalic acid sol–gel derived potassium doped titania supported cobalt oxide composite (5 g). Ethane flow rate (residence time within metathesis bed) is 2.0 sccm (2 min.) and 1.0 sccm (4 min).
EtBrðgÞ þ H2 OðgÞ þ MOðsÞ MBrðOHÞðsÞ þ EtOHðgÞ ð8Þ 2EtBrðgÞ þ MOðsÞ MBr2ðsÞ þ Et2 OðgÞ
ð9Þ
Prior to regeneration, this solid was treated with a stream of EtOH instead of EtBr under the same flow conditions. EtBr and ethylene were the only products observed for a period of more than 100 min, also consistent with this equilibrium picture. The equilibrium effect is less pronounced for copper based solids than for cobalt, but more side reactions (i.e. elimination and deep oxidation, both of which may go to completion [3]) are competitive over copper as well. Maximizing EtOH is therefore equally difficult for cobalt and for copper based solids, albeit for different reasons. An interesting feature of the potassium addition is that all of the potassium ions are expected to be associated with bromide counterions after several cycles of bromination/oxygenation, because reformation of K2O and Br2 equivalents from KBr reaction with O2 is not spontaneous. Even so, the presence of potassium bromide deactivates whatever sites are necessary for ether formation. We speculate that due to the electropositivity
In order to gain a better understanding of the redox and structural characteristics of the solid oxide/bromide exchange reagents used for ethane partial oxidation in this work, we examined the structure of the brominated and regenerated states of the solid. Fig. 5 shows XPS characterization of the zirconia supported copper oxide used for exchange in Fig. 2, at both stages in its cycle of use, bromide exchanged solid (grey), and regenerated solid (black). As suspected, based on the appearance and deep oxidation reactivity of the regenerated supported copper oxide material, the spectrum shows a classic cupric CuO pattern with 4 peaks due to splitting of the Cu 2p3/2 and 2p1/2 peaks [24,25]. The spent material shows a cuprous pattern consistent with both cuprous oxide and cuprous bromide [26,27] with single Cu 2p3/ 2 and 2p1/2 peaks. Besides the lack of the satellite peak associated with Cu(II) in the non-regenerated sample, XPS of the regenerated sample proves the successful reversibility of Cu(I)Br regeneration to Cu(II)O. As a complement to XPS data, which probes the first ˚ of the surface, elemental analysis was per10–50 A formed on the solids to evaluate the bulk composition.
18000
14000
CPS 10000
regenerated spent 6000 970
960
950
940
930
corrected binding energy, eV Fig. 5. XPS analysis of bromide exchanged (spent, grey line), and regenerated (black line) of oxalic acid sol–gel derived copper oxide supported on zirconia, clearly showing the oxidation and reduction of copper from cuprous to cupric states and vice versa during regeneration and metathesis with EtBr/HBr, respectively.
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Fig. 6. Powder XRD pattern of (a) as synthesized (b) exhaustively brominated and regenerated (6· as in Fig. 2) copper oxide, on zirconia (oxalic acid sol–gel prep); and (c) as synthesized and (d) exhaustively brominated and regenerated (3· as in Fig. 1) cobalt oxide on zirconia (oxalic acid sol–gel prep) metathesis reaction at 250 C. ( ): ZrO2 (JCPDS file: 86-1451), (þ): CuO (JCPDS file: 48-1548), and ( ): Co3O4 (JCPDS file: 42-1467).
Pore Volume in cc per gram
Regenerated CuO/ZrO2 contained 25.74% Cu and 46.40% Zr while the spent solid contained 21.89% Cu, 30.63% Zr and 27.25% Br. These measurements are in rough agreement with the CuBr/ZrO2 and CuZrO3 empirical formulae, before and after reaction. Overall, it appears that regenerated copper oxide is cupric and the spent material is cuprous. This observation is consistent with the deep oxidation products observed for these solids (Eqs. (6) and (7)). There appears to be a general correlation between facile regenerability and deeper substrate oxidation. The different redox capacities of transition metals in the metal oxides may be one reason for the differences in the product distribution between cobalt and copper oxides and this effect allows product distribution tuning. Structural and morphological differences of metal oxides may play a significant role as well. Fig. 6 shows powder X-ray diffraction patterns for freshly prepared samples and for those brominated and regenerated multiple times. The powder X-ray diffraction lines of metal oxides were indexed as CuO (JCPDS file: 48-1548), Co3O4 (JCPDS file: 42-1467) and ZrO2 (JCPDS file: 86-1451). The insensitivity of these patterns to multiple cycles of metathesis and reoxidation indicates high
hydrothermal stability of the metal oxides associated with the zirconia support. By using the Debye–Scherer formula for X-ray line broadening, the grain sizes for the materials were determined to be 12.4 nm and 15.4 nm, respectively for Co and Cu oxides on zirconia, respectively. BET analysis for the supported cobalt (206 m2/g) and copper (68 m2/g) oxides on zirconia demonstrates the higher surface area and porosity of the cobalt sample. The pore size distributions of both metal oxides indicate a large range of pore diameters (Fig. 7). SEM images of zirconia supported cobalt oxide (oxalic acid sol–gel) that had been taken through 30 cycles of exhaustive bromination and regeneration are shown in Fig. 8. Panel a shows the relatively featureless morphology of the solid (mostly supported CoBr2) after extensive reaction with EtBr/HBr and serves as a reference point for the other images. Upon regeneration, the solid, panel b appears to have a much coarser surface and a more porous structure. Upon imaging the regenerated material at higher magnification, small crystallites of 100 nm dimension are apparent, which are presumably of composition Co3O4 supported on zirconia. The EDS data (not shown) indicated that the surface compositions of the supported metal oxides consistent
Co3O4/ZrO 2
0.06 0.05 0.04 0.03 0.02
CuO/ZrO2
0.01 0 20
70
120
170
220
270
320
370
Pore Diameter in Angstroms
Fig. 7. BET analysis of oxalic acid sol–gel derived metal oxides after several cycles of metathesis and bromination/regeneration.
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213
Fig. 8. SEM micrographs of oxalic acid sol–gel prepared zirconia supported Co after 30 cycles of bromination/regeneration as described in Fig. 1: (a) 1000· magnification; unregenerated material with significant CoBr2 content. (b) 1000·; regenerated material, showing high porosity. (c) 85,000·; regenerated material, showing 100 nm crystallites of metal oxide.
mainly of cobalt (34%) and copper (35%), respectively, with very little zirconium (8.8%, and 4.4%, respectively) signal detected. 3.4. Effect of preparation method of metal oxide on activity High surface area is perhaps the most important property of a heterogeneous reagent or catalyst. In the case of heterogeneous bromine mediated ethane oxidation, persistant and robust reactivity over the course of multiple compositional and redox cycles between bromide and oxide is also important. Single use high surface area followed by collapse to a less reactive structure during regeneration is to be avoided. We compared our oxalic acid zirconia gel derived cobalt oxides with other preparations, including bulk CoO, wet impregnated Co on high surface area zirconia pellets, and citric acid gel cobalt/zirconia solid. The bulk cobalt oxide was reactive only in HBr neutralization up to 250 C but did not convert EtBr. The wet impregnated zirconia support was much more reactive than the bulk solid, while the citric
acid gel derived zirconia supported cobalt oxide was equally as reactive as the oxalic acid gel preparation. As mentioned above, the citric acid preparation showed no ether suppression and ethanol enhancement induced by potassium addition, while the oxalic acid preparation was subject to ether suppression by potassium.
4. Conclusion In summary, we have demonstrated that metal bromide/bromine mediated partial oxidation of ethane is possible at temperatures below 400 C. Ethane may be brominated with almost 90% selectivity for EtBr at 25% ethane conversion. The intermediate mixture of EtBr and HBr then may then be quantitatively neutralized and oxydebrominated by supported copper or cobalt oxides. In some cases further oxidation to products other than ethanol and diethyl ether or ethylene are observed. It is possible to prepare solid oxides with higher than bulk reactivity by varying supports and sol–gel synthesis parameters. Selectivity to ethanol is
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tunable by addition of alkali promoters to the supported solid. The bromide/oxide exchange reagents used in this process can be readily regenerated with complete bromine recovery. Comparison of bulk solids, mixtures prepared by wet impregnation, and sol–gel preparations of transition metal oxides showed that solids prepared by sol–gel techniques showed the highest capacity and most reproducibly regenerable reactivity for oxide/bromide exchange, and are thus the most promising candidates for use in a bromine mediated ethane partial oxidation process. Acknowledgment This research was supported by a sponsored research agreement between GRT Inc. and UCSB (PCF and GDS as PIs). References [1] G. Centi, F. Cavani, F. Trifiro, Selective Oxidation by Heterogeneous Catalysis, Kluwer, New York, 2001. [2] X.P. Zhou, I.M. Lorkovic, G.D. Stucky, P.C. Ford, J. Sherman, P. Grosso, US#6 472 572, 2002. [3] X.P. Zhou, A. Yilmaz, G.A. Yilmaz, I.M. Lorkovic, L.E. Laverman, M. Weiss, J. Sherman, E.W. McFarland, G.D. Stucky, P.C. Ford, Chem. Commun. (2003) 2294. [4] I.M. Lorkovic, M.L. Noy, M. Weiss, J. Sherman, E.W. McFarland, G.D. Stucky, P.C. Ford, Chem. Commun. (2004) 566.
[5] S. Sun, I.M. Lorkovic, M. Weiss, J. Sherman, E.W. McFarland, P.C. Ford, G.D. Stucky, Chem. Commun. (2004) 566. [6] K.M. Sundaram, M.M. Shreehan, E.F. Olszewski, Kirk-Othmer Encyclopedia of Chemical Technology, vol. 2004, Wiley Interscience, New York, 2001. [7] J.E. Murtagh, Energy Biomass Wastes 9 (1985) 1029. [8] A.N. Shigapov, N.R. Hunter, H.D. Gesser, Catal. Today 42 (1998) 311. [9] H.C. Anderson, E.R. Van Artsdalen, J. Chem. Phys. 12 (1944) 479. [10] I. Akhrem, A. Orlinkov, L.V. Afanaseva, E.I. Mysov, M.E. Volpin, Tetrahedron Lett. 36 (1995) 9365. [11] A. Yilmaz, G.A. Yilmaz, unpublished observations. [12] G.A. Yilmaz, I.M. Lorkovic, unpublished observations. [13] M. Weiss, S. Sun, I.M. Lorkovic, in preparation. [14] R.P. Bell, M.H. Ford-Smith, J. Chem. Soc. (1961) 1413. [15] M. Zapan, Theses Faculte 33 (1930) 9. [16] H.C. Anderson, G.B. Kistiakowsky, E.R. Van Artsdalen, J. Chem. Phys. 10 (1942) 305. [17] M. Guseinov, Nauk Azerbaidzhan. S.S.R. (1957) 63. [18] Y.G. Mamedaliev, M. Guseinov, Uchenye Zapiski Azerbaidzhan. Gosudarst. Univ. Im. M.S. Kirova (1957) 57. [19] D.M. Golden, S.W. Benson, Trans. Farad. Soc. 66 (1970) 2794. [20] M.W. Chase Jr., Journal of Physical and Chemical Reference Data, Monograph 9 (1998) 9. [21] J.L. Holmes, F.P. Lossing, J. Am. Chem. Soc. 110 (1988) 7343. [22] Thermodynamics Research Center Database. [23] M.S. Wong, H.C. Huang, J.Y. Ying, Chem. Mater. 14 (2002) 1961. [24] R.P. Vasquez, Surf. Sci. Spect. 5 (1998) 262. [25] D.V. Cesar, C.A. Perez, M. Schmal, V.M.M. Salim, Appl. Surf. Sci. 157 (2000) 159. [26] R.P. Vasquez, Surf. Sci. Spect. 5 (1998) 257. [27] R.P. Vasquez, Surf. Sci. Spect. 2 (1993) 144.
Bromine mediated partial oxidation of ethane over nanostructured zirconia supported metal oxide/bromide Aysen Yilmaz a, Xiao-Ping Zhou a, Ivan M. Lorkovic a,*, Gurkan A. Yilmaz a, Leroy Laverman a, Mike Weiss b, Shouli Sun b, Dieter Schaefer c, Jeffrey H. Sherman b, Eric W. McFarland c, Peter C. Ford a,*, Galen D. Stucky a,* a
Department of Chemistry and Biochemistry, University of California, Santa Barbara, CA 93106, USA b GRT Inc., 861 Ward Dr., Santa Barbara, CA 93111, USA c Department of Chemical Engineering, University of California, Santa Barbara, CA 93106, USA Received 8 September 2004; received in revised form 3 November 2004; accepted 3 November 2004 Available online 6 January 2005
Abstract Partial oxidation of ethane to an array of products was achieved through use of bromine/supported metal bromide as redox mediator for oxygen activation. In a two stage flowing fixed bed reactor, ethane bromination (350–450 C) to EtBr and HBr was followed by metathesis of these intermediates with supported cobalt or copper oxides (200–250 C) to produce ethanol (up to 64% selectivity at 45% EtBr conversion), diethyl ether (64% selectivity at 44% conversion), and ethylene (26% selectivity at 68% conversion at 200 C, but conversions and selectivities approaching 100% at temperatures >300 C) as major products, and butadiene, vinyl bromide, acetaldehyde, ethyl acetate, and acetone as minor products. The supported cobalt and copper bromides thus formed were reoxidized with flowing O2 to liberate bromine and this cycle was repeated at least three (and as many as fifty) times to establish the reproducibility of both the metathesis product distribution and of the oxide delivery and bromide sequestration capacity of the metal oxide/metal bromide mediators. Different formulations of the supported and unsupported metal oxide/metal bromide were investigated; zirconia or titania-supported Cu and Co oxides prepared from oxalic acid gels showed higher and more reproducible oxide/bromide exchange activity and capacity. These materials performance is discussed in terms of their morphology, crystallinity, and porosity at different states of bromination in the partial oxidation cycle. 2004 Elsevier Inc. All rights reserved. Keywords: Ethane; Partial oxidation; Bromine mediated; Supported metal oxide; Solid reactant; Ethanol derivatives
1. Introduction Direct conversion of light alkanes in natural gas feedstock to liquid products is an attractive research area in the context of more economical feedstock transportation, development of more energy efficient methods for *
Corresponding authors. Tel.: +1 805 893 5239; fax: +1 805 893 4120. E-mail addresses: [email protected], [email protected] (I.M. Lorkovic), [email protected] (P.C. Ford), stucky@chem. ucsb.edu (G.D. Stucky). 1387-1811/$ - see front matter 2004 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2004.11.003
the conversion of light alkanes to commodity chemicals, as well as towards minimization of greenhouse gas emission [1]. We have previously reported a novel process for functionalization of alkanes by bromine mediated partial oxidation [2,3] and have also shown how this technique could be used to produce higher hydrocarbons from methane [4], and olefin oxides from olefins [5]. Ethane is the second most abundant hydrocarbon component of natural gas and is itself a valuable feedstock for ethylene production through steam cracking [6]. Derivatives of ethylene include ethanol, from solid acid catalyzed hydration, acetaldehyde and its
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condensation products, from Wacker chemistry, ethylene oxide, and acetic acid. Ethanol production from biomass fermentation provides a large source of ethanol as well [7]. Work directed towards ethanol production from direct ethane oxidation has shown promise but thus far only low conversions and selectivities have been demonstrated [8]. We describe herein the application of our bromine/metal oxide mediated technique for partial alkane oxidation to the challenge of direct ethane conversion to ethanol and other derivatives of ethane (Eqs. (1)–(4)), Scheme 1. MBr2 þ 1=2 O2 ! MO þ Br2
ð1Þ
C2 H6 þ Br2 ! C2 H5 Br þ HBr
ð2Þ
C2 H5 Br þ HBr þ MO ! MBr2 þ EtOH
ð3Þ
Overall : C2 H6 þ 1=2O2 ! EtOH
ð4Þ
Selectivity is a key parameter in any chemical process. For methane and ethane partial oxidation all C–H bonds are equivalent and hence the main goal is to perform only a ‘‘2 electron’’ oxidation on the substrate and to avoid overoxidation, leaving the other C–H bonds intact. In this sense, ethane bromination is a particularly effective means of selective ethane oxidation, as the rate of secondary bromination to the two isomers of C2H4Br2 is slightly slower [9] than the first bromination under mild conditions, so 90% selectivity of bromoethane product is possible at 25% conversion (Eqs. (1) and (5)). C2 H5 Br þ Br2 ! HBr þ ðxÞ1; 1 C2 H4 Br2 þ ð1 xÞ1; 2 C2 H4 Br2
ð5Þ
Further developments in catalytic electrophilic bromination are also anticipated to improve the selectivity for monobromination beyond that achievable using free radical chemistry. For example, Akhrem et al. have re-
EtH T1 Br2 EtBr + HBr M(O) T2
MBr2
T3 1/2 O2
EtOH Et2O, C2H4, etc. Scheme 1. Metal bromide/metal oxide mediated ethane partial oxidation.
ported that CBr4 Æ 2AlBr3 is a reactive electrophilic catalyst for ethane bromination under mild conditions (60 C) [10]. To complete the transformation to usable products, however, bromide equivalents in HBr and alkyl bromide must be exchanged for oxide, and bromine mediator subsequently recovered. The advantages of bromine over other halogens in this alkane partial oxidation scheme can be related to the favorable reduction potential of Br2 in relation to O2. Alkane bromination is only mildly exothermic (while iodination is ergoneutral and does not go to completion) and therefore more straightforward to perform on a large scale than alkane fluorination and chlorination. Br2 recovery by metal bromide reaction with O2 is also spontaneous for a wider array of materials formulations than corresponding chlorine recovery from chlorides because of the less positive reduction potential of Br2 (1.07 V vs NHE) in comparison to Cl2 (1.37 V) and O2 (1.23 V). In this scheme, there are two sources of undesirable deeper substrate oxidation: secondary bromination in the alkane activation step (Eq. (5)) and redox activity of the metathesis material. The first of these overoxidations gives a mixture of dibromoethane isomers that have unique properties in their metathetical reactivity with metal oxides. 1,1-dibromoethane leads almost invariably to the HBr elimination product vinyl bromide upon reaction [11], although metathesis products such as acetaldehyde are possible at below 250 C. 1,2-dibromoethane, on the other hand, tends to undergo disproportionation to CO2 and more reduced products such as ethylene [12]. This secondary bromination product distribution may be controlled to favor 1,1- or 1,2dibromoethane (vide infra) [13]. The other source of substrate overoxidation is metal oxide redox reactivity within the redox potential range (1.2 V) experienced by the solid during cycles of metathesis and regeneration. This range is defined by exposure to alkane derivatives during metathesis (reducing atmosphere), and to O2 during regeneration (oxidizing atmosphere). If the oxidation state of the solid changes within this potential range, deeper oxidation of ethane derivatives and more oxygen consumption during regeneration is expected. Such deep oxidation does indeed occur over supported copper and cobalt oxides tested here, and in general for first row transition metal oxides, vanadium and higher. Furthermore, the more reactive partially oxidized intermediates are degraded more rapidly than ethane substrate. An example of the redox reactivity expected for many supported transition metal oxides is shown below; in this case a two electron oxidation of ethanol to acetaldehyde coincides with metathesis (Eqs. (6) and (7)). Butadiene, acetone, and ethyl acetate are also observed as side products. 3MBr2 þ 2O2 ! M3 O4 þ 3Br2
ð6Þ
A. Yilmaz et al. / Microporous and Mesoporous Materials 79 (2005) 205–214
207
3C2 H5 Br þ 3HBr þ M3 O4 ! 3MBr2 þ 2EtOH þ C2 H4 O þ H2 O
ð7Þ
We characterize in this study the parameters governing the conversion and selectivity of metal bromidemediated ethane partial oxidation. First we describe thermal ethane bromination product distributions as a function of reactor temperature, space-time and packing. Subsequently we examine oxygenated product distributions obtained upon treatment of the intermediate brominated ethane stream with reactive and regenerable supported copper and cobalt metal oxides. These solids are unique in their ability to sequester and neutralize stoichiometric amounts of bromide, and quantitatively release bromine upon oxygenation at temperatures <350 C over at least 50 cycles. Finally we describe the oxide and bromide morphologies as characterized by XRD, SEM, BET, and TGA, to attempt to gain insight into the favorable reactivity of this unique set of sol–gel prepared solids. For EtBr/HBr metathesis reactivity, equivalent amounts of unsupported cobalt and copper oxides, cobalt and copper oxides wet impregnated on high surface area zirconia supports, as well as other sol–gel preparation routes gave materials with similar HBr neutralization capacity but less metathesis reactivity with EtBr.
2. Experimental 2.1. Reactor system and reaction conditions The reactor system used for ethane oxidation reactions is composed of two vertical cyclindrical reactors in series, Scheme 2 (the first either 1 or 0.3 cm diam. · 10 cm len. and the second 1 · 10 cm). The feed to the first reactor is an ethane/bromine mixture of variable ratio (between 10:1 and 3.2:1 Ethane:Br2) which reacts either in an empty heated tube or in a packed bed of 50 mesh borosilicate beads. The second reactor contains the supported metal oxide fixed bed, reactive toward oxide/bromide exchange. Samples were taken for GC analysis at any location; upstream of the first reactor (blank), between the first and second reactors (ethane bromination products), or after the second reactor (products of overall reaction). Independent reactions of monobromoethane and dibromoethanes over metal oxides were also performed in the second stage of the reactor system. All reactions were carried out at atmospheric pressure. A mass flow controller (Matheson) was used to control the flow rate of ethane. Bromine was introduced into the ethane stream by a temperature controlled (ice water for P(Br2) = 70 Torr, 20 C bath for P(Br2) = 180 Torr) bromine bubbler equipped with a bypass valve to allow controlled entrainment of bromine into the ethane gas feed. Metathesis of EtBr over
Scheme 2. Laboratory scale reactor flow diagram for bromine mediated partial oxidation of ethane (dimensions not to scale).
metal oxide formulations was studied over a temperature range of 150–350 C. at gaseous space velocities between 0.02 and 0.1 h1 [(g feed)(g solid)1 h1]. Reactive metal oxides were typically exhausted to 15–25% of their bromine carrying capacity [3] before regeneration. 2.2. Synthesis of metal oxides and characterizations The supported metal oxides described in this study, MxZryOz and KwMxZryOz (where the molar ratio of M to Zr is 1:1, and the molar ratio of K to M to Zr is 1:4:5), as well as the corresponding titanates, were prepared from aqueous solutions of metal nitrates (0.5 M) thoroughly mixed with a solution of zirconium tetrapropoxide (or titanium isopropoxide) in aqueous oxalic acid (1.0 M). After evaporation of the solvent at 110 C, the mixture was calcined at 500 C for 4 h. After cooling, the solid was ground in a mortar and stored for further use. For data reported here, the bromination reoxidation cycle was performed until the product distribution did not change from run to run (usually 3 cycles). For purposes of comparison, zirconia supported cobalt and copper oxides were prepared using the same preparation above, except using a citric acid (1.0 M as above) based zirconia gel. Also tested were cobalt oxide supported on an equal weight of preformed zirconia/silica (Grace-Davison ‘‘Davicat’’ SIZR 4700 zirconia on amorphous silica, 368 m2/g), prepared by wet impregnation/calcination (500 C). Finally, comparable beds of
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unsupported Co3O4 admixed with enough 50 mesh glass beads to achieve the same bulk bed volume as the other samples were prepared. X-ray powder diffraction patterns were obtained on a Scintag PADX diffractometer using Cu-Ka radiation at 45 kV and 35 mA. The crystalline domain sizes were determined from powder XRD patterns and scanning electron microscopy (SEM) images. Environmental scanning electron microscope images were carried out with a Philips XL-30 ESEM-FEG. The EDS system (Princeton Gamma Technology) was equipped with an intrinsic germanium detector with a resolution of 115 eV and an ultra-thin window for light element detection and used IMIX version 10.594 software. Gas adsorption data for BET surface area determinations and pore diameter measurements were obtained with a Micromeritics ASAP-2000 porosimeter. XPS measurements were carried out using a Kratos Axis Ultra system. Monochromatic Al-Ka radiation was used. The anode was operated at 8 kV and 30 mA and the pressure in the analysis chamber was about 2 · 106 Pa. The binding energies (BE) were corrected for charging by assigning the C(1s) signal to 284.6 eV. Elemental analysis was performed by Galbraith Laboratories in Knoxville, TN. 2.3. Regeneration conditions Regeneration of the supported metal oxide from the spent metal bromide was performed by passing oxygen at 320 C through the reactor which contained the supported metal bromide. Evolved bromine was trapped as sodium hypobromite in a 2 M NaOH trap and analyzed by UV (HP-8452A diode array UV–Vis Spectrophotometer, e(330 nm) = 301 M1 cm1) [14]. During regeneration, the degree of utilization of O2 was estimated to be >90% by monitoring the non-bromine gas efflux rate, which completely stopped until all the metal bromide was converted into oxide. 2.4. Analysis of products Ethane conversion was measured by GC. The output stream of the bromination reactor was passed through an aqueous alkaline trap to remove HBr, and mass flow controlled dimethyl ether internal standard and nitrogen diluent were added just upstream of the GC injection valves. Ethane bromination product selectivities were measured by 1H-NMR (Varian 200 MHz and 400 MHz, delay time between pulses >20 s), by passing the output of the first reactor through a CDCl3 trap (typically 6.0 g for trapping 2 mmol products, quantified by comparison to known CHCl3 internal standard). While significant ethane, ethylene, and HBr were lost to headspace by this technique, brominated hydrocarbons were trapped quantitatively.
For tandem bromination/metathesis experiments, gaseous products exiting the second reactor, which contained the HBr scrubber/oxygen bromine exchange medium, were diluted with a nitrogen stream and passed through a dead volume (200 ml) and analyzed by an HP-6890A type GC equipped with DB-FFAP, Porapak-Q 80/100, and HP-Plot Q columns connected to FID, TCD, and mass selective (HP-5973) detectors, respectively. Alternatively, these products, which included bromoethane, ethanol, diethyl ether, ethyl acetate, acetaldehyde, and 1,3-butadiene were collected in a cooled (dry ice/acetone) CDCl3 trap, and analyzed by 1H-NMR as described above.
3. Results 3.1. Thermal ethane bromination reaction The reaction of ethane with bromine has been studied by numerous researchers [9,15–19]. The reaction is propagated by bromine atoms reacting with ethane to give HBr and by ethyl radicals reacting with bromine to give ethyl bromide and bromine atoms. As can be seen in Table 1, the selectivity to ethyl bromide at 25% conversion in thermal ethane bromination experiments is relatively invariant (85–90%) under a variety of conditions between two minutes and one second space time and 300 C to 450 C. Secondary bromination of ethyl bromide is slightly slower than ethane bromination [9] and gives 1,1- and 1,2-dibromoethane, which have similar thermodynamic properties [20]. Because the 1-yl-bromoethyl radical is more stable than the 2-yl-bromoethyl radical by approximately 5 kcal/mol [21], however, 1,1-dibromoethane is the kinetically favored product in a gas phase free radical reaction. Our empty reactor results are consistent with this picture, and 1,1-dibromoethane is the dominant dibromoethane isomer by over a factor of ten, Table 1. With the addition of seemingly innocuous glass beads, however (400 lm, 40–60 mesh, McMaster-Carr sandblasting media), the secondary bromination product distribution shifts dramatically to favor 1,2-dibromoethane over 1,1-dibromoethane by over a factor of ten. If this isomerization reaction is catalyzed by the bead surface, this result is surprising considering the supposed thermodynamic equivalence of these species [22], and the expectation that such an isomerization should be reversible. We rule out the possibility that the glass beads are catalyzing the secondary bromination because if such bromination catalysis were competitive with the gas phase reaction, the overall selectivity to dibromoethane products would be expected to increase in the absence of any similar enhancement in the initial primary bromination step. To wit, no increase in the overall rate of bromination is observed in the
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209
Table 1 Ethane bromination: ethane conversion and selectivity to ethyl bromide, 1,1-dibromoethane and 1,2-dibromoethane as a function of temperature, reactor space time, and bed packing Temperature (C)
450 450 400 400 375 350 350 300 300 280
Space time (s)
1 3 1 3 3 27 27 29 29 230
Packing
Conversion (%)
None None None None None None Borosilicate beads None Borosilicate beads None
Selectivity (%)
23.5 25.0 23.0 25.0 24.9 25.5 27.7 26.0 25.7 25.9
EtBr
1,1-DBE
1,2-DBE
90.4 89.3 91.2 89.4 87.5 86.5 85.2 87.3 87.8 82.1
8.8 8.9 7.7 9.2 10.0 12.8 1.2 12.0 1.7 15.6
0.8 1.8 1.1 1.4 2.5 0.7 12.2 0.7 10.3 2.3
All data represent 100% bromine conversion. EtH flow rate is typically 5 sccm and bromine is added by entrainment through a temperature controlled bromine bubbler.
Table 2 Bromine/cobalt oxide mediated ethane partial oxidation product distribution: dependence on supported cobalt oxide method of preparation EtBr conversion (%)
Et2O (%)
EtOH (%)
Other products
Bulk Co3O4 CoO wet impreg. on SiO2/ZrO2 Citrate sol/gel
<5 25
0 15
0 2
50
30
7
Oxalate sol/gel
43
27
7
2% 4% 3% 4% 3% 4% 3%
ViBr ViBr C2H4 ViBr C2H4 ViBr C2H4
For each entry, a cylindrical 8 (length) · 1 cm (diameter) plug of solid representing 25 mmol of Co was used as the oxide/bromide metathesis reagent. Ethane flow was 5 sccm and the Br2 flow was 0.5 sccm, giving a total exposure of the solid to 6.2 mmol bromide both as HBr and R–Br. The integrated product output over 5 h is summarized. All samples shown quantitatively and stoichiometrically neutralize streams containing HBr, producing either supported or unsupported CoBr2 and water. The product outputs shown represent results of the third cycle or higher of metathesis/ regeneration. Selectivity of all products is calculated in terms of converted ethane.
presence of the glass beads. We are currently pursuing the source of these effects further, and exploring the scope and limitations of this isomerization reactivity [13]. 3.2. Sequential zone flow reactor (SZFR) experiments The three step bromine and metal oxide mediated oxidation scheme was tested using ethane bromination to generate the feed that was then passed into metal oxide bed for neutralization/metathesis. To this end, ethane at 15 psia was entrained with bromine in a bubbler immersed in ice and the mixture passed through a bromination reactor as described above to generate a stream of EtBr, HBr, and trace dibromoethane in excess ethane carrier. This stream was directly passed into the metathesis reactor for oxide/bromide exchange with the solid oxide, and the output evaluated by periodic GC analysis. The effect of the support and preparation conditions on the capacity, reactivity, and regenerability of cobalt oxides as metathesis reagents was evaluated by comparing the behavior of bulk Co3O4 to that of supported (wet
impregnation) and sol–gel prepared samples, Table 2. While all samples are capable of neutralizing HBr and releasing Br2 upon treatment with O2 over several cycles, the supported cobalt oxide, prepared by both wet impregnation and sol–gel routes, are reactive in oxide bromide exchange with both EtBr and trace dibromoethane feeds. Despite the higher surface area of the commercially available silica/zirconia support, and the high reactivity of such supported materials reported by Ying and coworkers [23] the most reactive materials for EtBr metathesis are those prepared by the sol–gel route, with both citric and oxalic acid gels showing comparable reactivity. Figs. 1 and 2 show the time dependent product distribution for the two stage reaction representing both bromination (375 C) and metathesis, using a bed of sol–gel zirconia supported cobalt oxide and copper oxide, respectively. The bromine to ethane feed ratio is 1:10. The major product for this reaction for both oxides is diethyl ether (Figs. 1, 2), and there is significant breakthrough of bromoethane reactant (but not of HBr) at a point well below the calculated bromine neutralizing capacity of the solid. Other oxygenated products include
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A. Yilmaz et al. / Microporous and Mesoporous Materials 79 (2005) 205–214 1.2
CuZrO3 1.6 1.4
1
C2 in
mmoles/h
Et2O C2H4
0.6
EtBr ViBr
0.4
mmol/h
1.2 C2 in EtOH
0.8
1
EtOH Et2O
0.8
C2H4 AcH EtOAc
0.6
EtBr CO2
0.4
CO2
0.2
0.2
0 0.00
0 0.00
1.00
2.00
3.00
4.00
5.00
6.00
time, hours 1.00
2.00
3.00
4.00
5.00
6.00
time, hours Fig. 1. Ethane partial oxidation by sequential bromination (375 C, EtH:Br2 = 10) and oxide/bromide metathesis (200 C) over oxalic acid sol–gel derived zirconia/cobalt oxide composite (5 g). Ethane flow rate (residence time within metathesis bed) is 5.0 sccm (0.7 min).
ethanol, acetone, acetaldehyde, ethyl acetate and CO2, while elimination products include ethylene, vinyl bromide, and butadiene. The supported copper oxide is more selective to ethanol than the supported cobalt oxide (Fig. 3). Butadiene production was unique to the cobalt oxide. CuZrO3, in addition to evolving more CO2 and ethylene, also produced more ethyl acetate and acetone. The production of CO2 dropped sharply after the first hour of the reaction and stayed constant after Cu(II) was reduced to Cu(I). The selectivity of all major products is given in Fig. 3 (grey and striped gray bars). Carbon material mass balance analysis for the reaction over the metal oxides indicated that some of the reaction products (5–15% depending on the solid and degree of EtBr exposure) were adsorbed to the surface. Although diethyl ether hydrolysis to ethanol downstream from the metathesis reactor is a feasible path to
Fig. 2. Ethane partial oxidation by sequential bromination (375 C, EtH:Br2 = 10) and oxide/bromide metathesis (200 C) over oxalic acid sol–gel derived zirconia /copper oxide composite (5 g). Ethane flow rate (residence time within metathesis bed) is 5.0 sccm (0.7 min). The spikes of ethanol are due to condensed aqueous ethanol entering the evaporation dead volume, upstream from the GC, which serves to dampen and allow quantitation of resulting short intervals of high ethanol reactor efflux.
ethanol formation, ether itself is not as desirable a product. In an effort to reduce the ether output of EtBr/HBr metathesis reactor, other solids and dopants were investigated. Fig. 3 shows the metathesis product output of undoped and potassium-doped cobalt and copper zirconates. The potassium-doped solid reactivity is approximately the same as the undoped materials, but the product distribution was significantly altered to favor EtOH over other products. Fig. 4 shows the temporal product distribution of the two stage bromine mediated ethane partial oxidation using potassium doped supported cobalt oxide at two slower flow rates, run to the same oxide conversion. Significantly, for these solids the EtBr breakthrough is observed at approximately the same level of solid utilization (5%), and is independent of the feed flow rate, Fig. 4, suggesting the gas/solid equilibrium idealized in Eqs. (8) and (9).
Fig. 3. Steady state (stabilized over several cycles) product distribution observed for oxalic acid sol–gel derived zirconia supported copper and cobalt oxides, unpromoted or promoted with potassium. Conditions are identical to those used in Figs. 1 and 2.
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2 cc/min EtH
mmol C2/h
1 C2 in
0.8
EtOH Et2O
0.6
C2H4 EtBr
0.4
ViBr CO2
211
of potassium and basicity of associated anions, these basic counterions (bromide/oxide) are responsible for deactivating electrophilic sites of the undoped metal zirconate. Ethylene production, on the other hand, is not as strongly affected by potassium addition as ether formation, suggesting that ethylene may form at either electrophilic or basic sites on the oxide. In an additional twist, the ether output of the citrate based cobalt/zirconia gels was not affected by the addition of potassium.
0.2 0 0.00
3.3. Characterization of metal oxides 2.00
4.00
6.00
8.00
10.00
12.00
time, h 1 cc/min EtH
mmol C2/h
1 C2 in EtOH Et2O C2H4
0.8 0.6
EtOAc EtBr ViBr CO2
0.4 0.2 0 0.00
5.00
10.00
15.00
20.00
time, h Fig. 4. Ethane partial oxidation by sequential bromination (375 C, EtH:Br2 = 10) and oxide/bromide metathesis (225 C) over oxalic acid sol–gel derived potassium doped titania supported cobalt oxide composite (5 g). Ethane flow rate (residence time within metathesis bed) is 2.0 sccm (2 min.) and 1.0 sccm (4 min).
EtBrðgÞ þ H2 OðgÞ þ MOðsÞ MBrðOHÞðsÞ þ EtOHðgÞ ð8Þ 2EtBrðgÞ þ MOðsÞ MBr2ðsÞ þ Et2 OðgÞ
ð9Þ
Prior to regeneration, this solid was treated with a stream of EtOH instead of EtBr under the same flow conditions. EtBr and ethylene were the only products observed for a period of more than 100 min, also consistent with this equilibrium picture. The equilibrium effect is less pronounced for copper based solids than for cobalt, but more side reactions (i.e. elimination and deep oxidation, both of which may go to completion [3]) are competitive over copper as well. Maximizing EtOH is therefore equally difficult for cobalt and for copper based solids, albeit for different reasons. An interesting feature of the potassium addition is that all of the potassium ions are expected to be associated with bromide counterions after several cycles of bromination/oxygenation, because reformation of K2O and Br2 equivalents from KBr reaction with O2 is not spontaneous. Even so, the presence of potassium bromide deactivates whatever sites are necessary for ether formation. We speculate that due to the electropositivity
In order to gain a better understanding of the redox and structural characteristics of the solid oxide/bromide exchange reagents used for ethane partial oxidation in this work, we examined the structure of the brominated and regenerated states of the solid. Fig. 5 shows XPS characterization of the zirconia supported copper oxide used for exchange in Fig. 2, at both stages in its cycle of use, bromide exchanged solid (grey), and regenerated solid (black). As suspected, based on the appearance and deep oxidation reactivity of the regenerated supported copper oxide material, the spectrum shows a classic cupric CuO pattern with 4 peaks due to splitting of the Cu 2p3/2 and 2p1/2 peaks [24,25]. The spent material shows a cuprous pattern consistent with both cuprous oxide and cuprous bromide [26,27] with single Cu 2p3/ 2 and 2p1/2 peaks. Besides the lack of the satellite peak associated with Cu(II) in the non-regenerated sample, XPS of the regenerated sample proves the successful reversibility of Cu(I)Br regeneration to Cu(II)O. As a complement to XPS data, which probes the first ˚ of the surface, elemental analysis was per10–50 A formed on the solids to evaluate the bulk composition.
18000
14000
CPS 10000
regenerated spent 6000 970
960
950
940
930
corrected binding energy, eV Fig. 5. XPS analysis of bromide exchanged (spent, grey line), and regenerated (black line) of oxalic acid sol–gel derived copper oxide supported on zirconia, clearly showing the oxidation and reduction of copper from cuprous to cupric states and vice versa during regeneration and metathesis with EtBr/HBr, respectively.
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Fig. 6. Powder XRD pattern of (a) as synthesized (b) exhaustively brominated and regenerated (6· as in Fig. 2) copper oxide, on zirconia (oxalic acid sol–gel prep); and (c) as synthesized and (d) exhaustively brominated and regenerated (3· as in Fig. 1) cobalt oxide on zirconia (oxalic acid sol–gel prep) metathesis reaction at 250 C. ( ): ZrO2 (JCPDS file: 86-1451), (þ): CuO (JCPDS file: 48-1548), and ( ): Co3O4 (JCPDS file: 42-1467).
Pore Volume in cc per gram
Regenerated CuO/ZrO2 contained 25.74% Cu and 46.40% Zr while the spent solid contained 21.89% Cu, 30.63% Zr and 27.25% Br. These measurements are in rough agreement with the CuBr/ZrO2 and CuZrO3 empirical formulae, before and after reaction. Overall, it appears that regenerated copper oxide is cupric and the spent material is cuprous. This observation is consistent with the deep oxidation products observed for these solids (Eqs. (6) and (7)). There appears to be a general correlation between facile regenerability and deeper substrate oxidation. The different redox capacities of transition metals in the metal oxides may be one reason for the differences in the product distribution between cobalt and copper oxides and this effect allows product distribution tuning. Structural and morphological differences of metal oxides may play a significant role as well. Fig. 6 shows powder X-ray diffraction patterns for freshly prepared samples and for those brominated and regenerated multiple times. The powder X-ray diffraction lines of metal oxides were indexed as CuO (JCPDS file: 48-1548), Co3O4 (JCPDS file: 42-1467) and ZrO2 (JCPDS file: 86-1451). The insensitivity of these patterns to multiple cycles of metathesis and reoxidation indicates high
hydrothermal stability of the metal oxides associated with the zirconia support. By using the Debye–Scherer formula for X-ray line broadening, the grain sizes for the materials were determined to be 12.4 nm and 15.4 nm, respectively for Co and Cu oxides on zirconia, respectively. BET analysis for the supported cobalt (206 m2/g) and copper (68 m2/g) oxides on zirconia demonstrates the higher surface area and porosity of the cobalt sample. The pore size distributions of both metal oxides indicate a large range of pore diameters (Fig. 7). SEM images of zirconia supported cobalt oxide (oxalic acid sol–gel) that had been taken through 30 cycles of exhaustive bromination and regeneration are shown in Fig. 8. Panel a shows the relatively featureless morphology of the solid (mostly supported CoBr2) after extensive reaction with EtBr/HBr and serves as a reference point for the other images. Upon regeneration, the solid, panel b appears to have a much coarser surface and a more porous structure. Upon imaging the regenerated material at higher magnification, small crystallites of 100 nm dimension are apparent, which are presumably of composition Co3O4 supported on zirconia. The EDS data (not shown) indicated that the surface compositions of the supported metal oxides consistent
Co3O4/ZrO 2
0.06 0.05 0.04 0.03 0.02
CuO/ZrO2
0.01 0 20
70
120
170
220
270
320
370
Pore Diameter in Angstroms
Fig. 7. BET analysis of oxalic acid sol–gel derived metal oxides after several cycles of metathesis and bromination/regeneration.
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Fig. 8. SEM micrographs of oxalic acid sol–gel prepared zirconia supported Co after 30 cycles of bromination/regeneration as described in Fig. 1: (a) 1000· magnification; unregenerated material with significant CoBr2 content. (b) 1000·; regenerated material, showing high porosity. (c) 85,000·; regenerated material, showing 100 nm crystallites of metal oxide.
mainly of cobalt (34%) and copper (35%), respectively, with very little zirconium (8.8%, and 4.4%, respectively) signal detected. 3.4. Effect of preparation method of metal oxide on activity High surface area is perhaps the most important property of a heterogeneous reagent or catalyst. In the case of heterogeneous bromine mediated ethane oxidation, persistant and robust reactivity over the course of multiple compositional and redox cycles between bromide and oxide is also important. Single use high surface area followed by collapse to a less reactive structure during regeneration is to be avoided. We compared our oxalic acid zirconia gel derived cobalt oxides with other preparations, including bulk CoO, wet impregnated Co on high surface area zirconia pellets, and citric acid gel cobalt/zirconia solid. The bulk cobalt oxide was reactive only in HBr neutralization up to 250 C but did not convert EtBr. The wet impregnated zirconia support was much more reactive than the bulk solid, while the citric
acid gel derived zirconia supported cobalt oxide was equally as reactive as the oxalic acid gel preparation. As mentioned above, the citric acid preparation showed no ether suppression and ethanol enhancement induced by potassium addition, while the oxalic acid preparation was subject to ether suppression by potassium.
4. Conclusion In summary, we have demonstrated that metal bromide/bromine mediated partial oxidation of ethane is possible at temperatures below 400 C. Ethane may be brominated with almost 90% selectivity for EtBr at 25% ethane conversion. The intermediate mixture of EtBr and HBr then may then be quantitatively neutralized and oxydebrominated by supported copper or cobalt oxides. In some cases further oxidation to products other than ethanol and diethyl ether or ethylene are observed. It is possible to prepare solid oxides with higher than bulk reactivity by varying supports and sol–gel synthesis parameters. Selectivity to ethanol is
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tunable by addition of alkali promoters to the supported solid. The bromide/oxide exchange reagents used in this process can be readily regenerated with complete bromine recovery. Comparison of bulk solids, mixtures prepared by wet impregnation, and sol–gel preparations of transition metal oxides showed that solids prepared by sol–gel techniques showed the highest capacity and most reproducibly regenerable reactivity for oxide/bromide exchange, and are thus the most promising candidates for use in a bromine mediated ethane partial oxidation process. Acknowledgment This research was supported by a sponsored research agreement between GRT Inc. and UCSB (PCF and GDS as PIs). References [1] G. Centi, F. Cavani, F. Trifiro, Selective Oxidation by Heterogeneous Catalysis, Kluwer, New York, 2001. [2] X.P. Zhou, I.M. Lorkovic, G.D. Stucky, P.C. Ford, J. Sherman, P. Grosso, US#6 472 572, 2002. [3] X.P. Zhou, A. Yilmaz, G.A. Yilmaz, I.M. Lorkovic, L.E. Laverman, M. Weiss, J. Sherman, E.W. McFarland, G.D. Stucky, P.C. Ford, Chem. Commun. (2003) 2294. [4] I.M. Lorkovic, M.L. Noy, M. Weiss, J. Sherman, E.W. McFarland, G.D. Stucky, P.C. Ford, Chem. Commun. (2004) 566.
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